2  Equipment 

	   2.1	Runway.
	   2.2	Short Runways
	      2.2.1  Bumps versus the tow vehicle.
	      2.2.2  Runway lights versus the glider.
	      2.2.3  Pulley Launch.     
	   2.3	The Glider.
	   2.4	Tow vehicle.
	   2.5	Release hitch.
	   2.6	Rope choice.
	      2.6.1  Testing
	      2.6.2  Rope Longevity
	      2.6.3  Rings
	      2.6.4  Knots and splices
	      2.6.5  Weak link
	      2.6.6  Flags on the launch rope
	      2.6.7  Rope handling
           2.7  Release mechanism

2.1  Runway


The runway is indeed "a piece of equipment," of the most basic sort.
The ideal is a 5,000 foot long paved runway, as this is better for the
tow vehicle.  (Wear and tear on the tow vehicle is important at least
to its owner.)


2.2  Short Runways


A shorter paved runway with grass over-runs at one or both ends is
quite satisfactory.  My experience is that 4,500 feet of linear space
is really a minimum for consistently good launches.  The drawbacks of
the shorter runway are:


2.2.1  Bumps versus the tow vehicle.


There is often a "step" at the end of the runway, a bump over which
the tow vehicle must pass, under power, at 30 to 45 miles per hour.
This is hard on CV joints and erodes the owner's kindly feelings.


2.2.2  Runway lights versus the glider.


Runway lights usually are positioned at the ends of the pavement as
well as the sides.  These are a minimal hazard, as they break away
when hit, but do make little dents in the truck or glider that are
cosmetically troublesome, especially to the owners, and make it hard
to deflect blame.  Breaking them is costly, and scores no points with
the airport manager.  There is, however, always a gap in the middle
through which the tow vehicle can be driven when necessary.  A mid to
high wing glider such as the Blanik or Schweizer 2-33 can taxi through
them in an urgency, with care to make sure there's enough speed to
maintain aileron effectiveness.

Umm.. I remember sitting in the spectator seat of my L-13 once as a
friend rolled out after landing.  "I don't think you're going to make
it through the end lights!"  "Naw, we'll make it!"  "The ailerons
aren't going to be working."  "Sure, they're fine!"  Crunch.  "Sorry."
About $200 sorry, and that was only the light; we didn't fix the
tiny ding in the L-13's wing, one of many.  If we taxi through and over
runway lights, we need to have enough airspeed that the ailerons can
lift the wings over them.

It is possible to make ground launches from the grass over-run,
leading the rope through the end lights.  We have done this hundreds
of times.  But it should be done only if


 -  The glider's wings are high enough to clear the lights comfortably
    while taxiing;

 -  The glider is at a sufficiently high speed when going past
    (through) the lights such that there is full control
    effectiveness; ideally it's already well off the ground;

 -  There is enough room, either on landing after touchdown or on
    takeoff, to bring the glider to a complete stop before reaching
    the lights, with the use of brakes;

 -  The brakes are effective enough to be trusted.

 -  The PIC is fiscally able to endure the consequences to the glider
    and the lights of an error in judgment.

Short runways (less than 4,500 ft., in my judgment) require the pulley
technique.  As I have no experience with this technique, I do not
have any advice for you.

The alternative to the pulley technique is the "elastic launch." I saw
this used by a club in south-central Iowa a few years ago.  They had
only about 75 feet of grass overrun at the end of the runway, a
polypropylene rope (lots of stretch), and a spavined old Buick without
much low-end torque (any more).  ("Elastic launch" is my term...)

The drill was to put the glider at the edge of the corn field pointing
down the runway centerline, lead the rope through the center of the
end-marker lights, and park the tow car well down the runway, off to
the side, with lots of slack in the rope.  
About 200 feet of slack, actually.

When all was ready, the wing runner waved the big flag widely, the tow
car's driver floored the accelerator and carved a turn out onto the
runway, joining up with the centerline and hustling fast into the
wind.

When the tow car got up to about 30 miles an hour, the glider pilot
felt a powerful surge of acceleration -- but not a sharp jerk, thanks
to the line's elasticity.  The wing runner took about two steps and
the wing was snatched from his hand.  The glider was aloft after only
about 15 feet of ground roll, and over the lights.

Now the principle of elasticity finished its cycle, and the rope went
slack with the glider just barely above stall speed and about a dozen
feet off the runway.  If the pilot pulls up too gently, the glider
mushes down more or less firmly onto the runway and gets snatched
again.	If the pilot pulls up too steeply, the glider stalls and
falls, possibly injuring itself in the bottom and maybe the pilot,
too, who knows?  But if the pilot pulls up just enough, the slack is
taken up by the beginning of the climb, the car doesn't stall and
keeps accelerating, and the launch is successfully under way.

This was how I, the passenger, was introduced to auto tow launching,
and while it was enormous fun, now that I understand what was
happening mechanically and aerodynamically I believe it to be far too
risky to be part of a training operation.  It belongs in the circus.


2.2.3  Pulley Launch


This technique requires the use of a rope of double the length needed
for straight autotow launching.  A "anchor" vehicle is placed at the
far end of the runway, and one end of the rope attached to it.	A
sturdy pulley, usually made out of an old steel wheel and an axle, is
attached to the tow vehicle begins the launch at the anchor vehicle
and travels toward the glider.	The pulley provides a 2:1 purchase
that effectively doubles the speed of the tow vehicle.

An advantage of this technique is that the ends of the rope can be
"swapped," reducing the time spent in retrieving the rope (assuming
that the rope falls from the glider onto an accessible part of the
airport, an unreasonable assumption in crosswind conditions).

I don't have any experience with this directly.  I've heard a talk
from pilots in the Southern US who have used this successfully; I've
also heard from a pilot who saw a pulley launch done wrong -- the tow
car went "full speed" instead of "half speed," and it pulled the wings
off the glider (just as a winch launch can do) and he saw his friend
die.  So it has to be done right.  I'll let someone else tell you how
to do it safely.


2.3  The Glider


It is a pretty good idea to use a glider that's approved for ground
launching by the manufacturer.	I haven't researched which are and are
not, but would be very reluctant to use one that had only a nose hook
for towing, without the specific approbation of the manufacturer.

A CG hook is preferred to a nose hook, as ground launching with a nose
hook places excessive strain both on the nose hook (a downward force
vector that it may not be designed for) and on the empennage
(continuous maximum nose-up pitch force to counteract the downward
pull on the nose).  Having said this, I know that in Ames, IA, the
club has been auto-launching their Blanik for years using the nose
hook without any problems of which I'm aware.

My own experience is that using the CG hook gets much better launches;
the altitude and rate of climb are much easier to control with the CG
hook.  Using the nose hook, basically the technique is just "full back
stick" all the way up, and the climb is very gently, the maximum
altitude rather limited.  Using the CG hook, the climb can be about as
steep as you want within the strength of the weak link, and full
elevator effectiveness is preserved throughout the climb.  I have
observed that the greater elevator control with the CG hook does make
PIO a little more likely for the nervous, inexperience pilot during
climbout.  More on this later; it's easy to eliminate or control by
making corrections slowly; often be swift, never be sudden.


2.4  Tow vehicle


A powerful pickup truck is ideal.  In my experience, the more power
the better.  This means using a "full-sized" pickup with at least a
V-8 engine.  Four wheel drive is not necessary except if pulling from
a grass runway (grass is very slippery).  An automatic transmission is
a must.  One shift is necessary during the tow, and few drivers are
able to shift a manual transmission with the alacrity required to
continue the tow safely, without risking damage to the transmission.
In addition, a mechanical clutch is not able to handle repeated heavy
starting loads, so premature replacement may be necessary.

The tow vehicle and the hitch assembly should be approved by the
manufacturers for pulling greater than 2-ton loads, as the rope you
should use has an actual breaking strength of almost 2000 lbs.	You
want the rope to break before the hitch or the bumper comes off the
truck...

As I mentioned, my first autotow experience was at the hands of a
defunct Iowa club, which used a spavined old automobile, with the roof
cut off by a welding torch for better visibility.  This car didn't
have enough torque to launch the glider effectively from a dead stop,
so they snatched it up using the elasticity of their polypropylene
rope.  The piloting challenge was to recognize whether the glider had
enough airspeed to, without stalling, take up the slack by pitching
up.  This had to be done quickly, as the car didn't accelerate well
enough to take up the slack on its own before the glider stalled.

Do not copy this technique.  The tow vehicle must have enough torque
to safely launch the glider from a dead stop, or you don't have enough
margin for reliably safe launches.  Having said this, the launch is
much safer, in my judgment, if there's enough slack in the line to let
the tow vehicle reach about 12-15 miles an hour.  With a 1500-foot
dacron rope, there's enough elasticity in the weave and the polymer to
prevent a jerk, the tow vehicle is off the weak part of the torque
curve, and the glider gains control effectiveness after 75 - 100 feet
of ground roll instead of 150-300, which makes the launch safer and
more predictable.  Rebound slack does not develop with dacron, at this
speed.

It is fair to pay the tow vehicle's owner something for each tow, as
this whole business adds extra wear to the engine and drive train.


2.5  Release hitch


Go to the Boys' Toy Store and pick up a hitch in the spare parts
department, and a strap hinge that will fit it.  Buy a small eye bolt
that will fit through one of the holes in the hinge and nuts to hold
it on.

From the Schweizer Aircraft Company or Wings and Wheels buy a release
hook.

Now put your pieces together, using dabs of 1-minute epoxy to fasten
them in place.	Epoxy holds securely, but can be broken free easily if
you make a mistake.  The epoxy will burn away when the pieces are
welded in place.

Mount the eye bolt on the hinge, on the middle of one side of the
strap, with the eye perpendicular to the long axis of the strap.
Mount this strap of the hinge on the top surface of the hitch, with
the hinge just off the back edge of the hitch.	The hinge itself
should be oriented so that the free strap folds up onto the hitch.

Then mount the Schweizer release on the free side of the strap hinge,
on top of the strap, with the release lever and the release opening on
the side toward the hinge.  Make sure that it's out far enough on the
strap so that the release can open completely and freely, without
being stopped by impinging on the hinge itself.

The free side of the hinge should be reinforced with a small metal
plate welded underneath the hitch to form a T.

Now (get out your arc-welding equipment or) take your assembled parts
to a welder and have the parts welded in place.  Weld the hinge tangs
to the strap, so that they can't open up.  If this is not done, during
one of the first few tows, the metal holding the hinge pin will simply
unwrap and the hinge will fall apart, as we discovered just one tow
before it happened.

Last, take a piece of string long enough to reach from the hitch to
the interior of the tow vehicle's cab, tie it onto the release's latch
bar, and pass it through the eye bolt so that you have a tow release
for the driver.  Frankly, you can't see the glider well enough from
the truck to tell it's in trouble in time to cut it loose before
trouble actually develops -- remember, the glider is 1500 feet away.
But you do want to release the rope's lower end immediately after the
glider releases, as this makes the retrieve ever so much easier.

The hitch assembly is now ready for use.

Here are two illustrations of the completed hitch assembly:  First, with
the hitch open and second, with the hitch closed. (Click on the highlighted word;
the illustration will open in a new browser window.)


2.6  Rope choice


We ordered dacron rope from David Bradley, C.S.R. Braided Products
Division, 330 Airport Road, Lehighton, Pennsylvania, 18235
610-379-5666 (bradley@voicenet.com)  You want the longest rope he can
provide, at least 1500 feet.  Dacron braided rope is, I feel, the best
compromise in performance, ease of handling and repair, and cost.

Polypropylene (yellow) rope can be used also.  It has more elasticity,
and so is more prone to pilot-induced oscillation and is more prone to
generate slack just as you need to start the initial climb.  It is
more prone to UV damage.  I believe it is cheaper.

It is tempting to use jacketed yacht braid, as the longevity should be
much greater.  But because the cost is many times higher and splicing
is a chore, I have not tried this.


2.6.1  Testing


I strongly recommend that you test your rope's actual breaking
strength with a tensiometer if possible.  My experience is that the
3/8 inch dacron rope we buy from David Bradley has a breaking strength
when new of about 1800 lb, and that when the strength decreased
through wear to less than 1200 pounds we begin experiencing excessive
rope breaks.


2.6.2  Rope Longevity


The rope lifespan depends mostly on how much towing you do and what
are the ground conditions at your field.  Abrasion is the biggest
factor.  Part of the abrasive damage is from being dragged across the
ground.  Hidden abrasion and weakening occurs from particles of dirt
that are picked up in the fibers, that cut or nick the fibers as the
rope is worked, especially when tension is applied.  Rope will last
much longer when used on a grass strip that doesn't have patches of
dirt.  Rope life will be shortest when used from a gravel runway
because of all the sand that is picked up.

The greatest wear is at the truck end of the rope.  This part of the
rope is the last to leave the ground during tow, and experiences the
most stress when being dragged back to the launch point.  My
experience is that most rope breaks occur within the three hundred
feet of rope closest to the tow vehicle.

We don't swap rope ends to even out wear, because it's desirable to
have rope breaks occur close to the truck rather than close to the
glider.  The weight of the rope hanging from the glider helps ensure
that the rope actually releases when the yellow handle is pulled, and
in the Blanik, ensures that the round launch bridle drops away when
released, rather than wrapping itself around the empennage, as it does
when the weak link is located at the front of the bridle.


2.6.3  Rings


The Schweizer release used on the tow hitch of course requires a
Schweizer ring.  At the glider end, use what the glider needs.	If you
use the Blanik's nose hook, a special ring is needed, smaller than a
Schweizer and bigger than a Tost.  If you have more than one glider
type, use a Schweizer ring at both ends and put the weak link for
other gliders on a carabiner that can be quickly snapped onto the
Schweizer ring.


2.6.4  Knots and splices


Don't tie knots in the rope: this halves its strength.	Splicing
braided rope is easy, taking advantage of the finger-trap principle to
avoid slippage.

Buy a set of quarter-inch diameter hollow aluminum knitting needles,
and use a pipe cutter to cleanly nip off the end caps.	This makes a
lovely set of splicing fids, and it's easy to find spares.

Splices should be made by running at least three feet of rope, each
end inside the other.  Weave the rope in and out through this length.
This is not quite as strong as unspliced rope, and wears faster, but
allows you to preserve most of the rope's length as it ages.

The rings should be attached with a wear-resistant loop.  To make
this, run the fid, pointing toward the bitter end, through the center
of a 3-inch section of rope, 18 to 30 inches from the end.  Then slip
this into the ring so that the section of the rope through which the
fid passes is in the ring.  Bring the bitter end around and fit it
into the fid.  Pull the fid and rope through, making a loop to hold
the ring.  You will notice that this makes a loop of rope forming a
jacket for itself.  Now thread the fid up the rope from the ring and
loop, and nest the loose end of the rope into itself.  This makes a
strong, simple and abrasion-resistant fixture for the ring, and does
not reduce the rope's strength as a knot would.


2.6.5  Weak link


The weak link is required to have a breaking strength of 80% to 200%
of the maximum gross weight of the glider.  This applies to both
aerotow and ground launch applications.  Anything less than about 115%
of the max gross weight is just too weak for autotow launching without
excessive rope breaks, in my experience.

We tested our tow ropes, and found their strength when new to be about
1800 lb., less than 200% of the Blanik L-13's MGW of 1100 lb.  We then
dispensed with the weak link, and have found this to be preferable to
having one.

I recommend that you talk to the industrial engineering department of
the closest college and find a tensiometer with which to test your
ropes.	This will open your eyes.

One advantage of having a weak link is to control the point at which
breaks occur.  Once some wear occurs on the tow rope, nearly all
breaks occur down near the tow vehicle.  We have learned to prefer
this for the following reasons:


 -  The Blanik L-13 has a ground launch bridle that consists of two
    twenty to twenty five foot lengths of sturdy rope with a metal
    link at the glider end, joined to a ring that attaches to the tow
    rope,  The weak link is traditionally put between the tow rope and
    the bridle.

    Our experience is that when this weak link breaks, the bridle
    almost always wraps itself around the horizontal stabilizer.  The
    pilot feels a little flutter from the elevator.  This is probably
    not a dangerous flight condition, but the bridle then falls off
    unpredictably.  This means the loss  of the 200 dollar bridle, and
    risks it falling and causing damage to persons or property, not a
    risk I wish to take.  We lost one in a corn field one day...

 -  The weight of the rope helps pull the bridle away from the glider,
    and causes it to land on the airport in the runway environment
    immediately after the break.

 -  The windage of the rope pulls the bridle backward, causing the
    bridle to back-release, sometimes even before the pilot is ready
    to pull the release handle.  If the lengths of rope on the sides
    of the bridle are slightly unequal, one side will release before
    the other.	This causes no change at all in the way the glider
    handles, and the risk is that the pilot will hear the click and
    assume that the rope is gone, and not bother to pull the release.
    Normally the rope will eventually back-release, but...if the hook
    gets pulled sideways for some reason it could jam, I think.  So
    when you hear one click, be sure you pull the release fully and
    firmly anyway.

 -  The bridle and the rope are much easier to find than the bridle
    alone.  (The wind does blow them along...)


2.6.6  Flags on the launch rope.


At the conclusion of a launch, the rope drifts along with the wind.
As there's usually some crosswind component, the rope is seldom on the
runway after it falls, unless the pilot has flown upwind during the
launch by just the right distance.  If the pilot forgets to correct
for the crosswind component or guesses badly, the rope may drift two
or three hundred feet sideways, especially after a rope break.

Before the launch, consider the worse case: where might the rope fall
if the climb is badly managed?	Across a row of parked airplanes?  On
the FBO?  Across a factory nearby?  Across security fencing topped
with razor wire?  Across a swamp?  Through a woods, or worse, through
a brier patch?	Sometimes it works to simply pull the rope from one
end, but it doesn't work well to pull the ground launch bridle through
a hostile environment.	Hard on the bridle; hard on the factory, the
airplanes, the chain link fencing, or the construction equipment and
supplies.  We've seen all of these happen except that we didn't ever
drop the rope across an airplane.  But on some launches, into
crosswinds, with some of our student pilots, I confess I was deeply
concerned that the launch be flown just right.  We didn't ever
damage anything by dropping the rope and bridle off the airport,
but this was not luck, it took planning and care.

I have found that sewing large flags onto the ground launch bridle is
invaluable in locating it after release.  They make it visible as it
falls and after it's disappeared into the tall grass.  I went to a
fabric store and simply bought two yards of the brightest, most sturdy
cloth they had (gold colored, in this instance), then persuaded my
spouse to sew them onto the ground launch bridle.  Click for a photo of 
the flags in use.

The only disadvantage to using flags is that they create windage, and
after leveling off at the top of the climb, after tension is off the
rope, this causes the bridle to back release, sometimes only one side
releases.  The only clue to back release is that a faint "click" is
heard.

It's necessary to attach a flag to each side of the bridle, as
otherwise the windage of the side with the flag will always cause
asymmetric back release.

This windage is a reason to attach the flags as close to the end of
the rope or bridle as possible, as they will cause the rope to bow in
the slipstream and to back release.

It's important, in the Blanik L-13, to pull the release fully, and to
hold it open for a couple of seconds, to give time for a sticky clip
to be freed by traction and vibration and pull away.  A quick pull and
release or multiple quick pulls and releases may not allow it to drop
away, and there is no clue in the glider's handling that the rope is
still attached.

It is helpful for the tow vehicle to confirm visually that the rope
has dropped away after release and to radio the glider to provide
assurance of  this.  Attaching flags to the bridle (or rope) is
essential to permitting the release to be seen.  Back-release failure
is rare, but does happen, and is easily remedied by pulling the
release and holding it open.  The release mechanism is so sturdy and
simple that I can't imagine a total failure unless it's been
maintained carelessly.


2.6.7  Rope handling


I've used a garden-hose reel to roll up and store the rope.  These are
readily available at home, garden, and discount stores for about $25.
The larger ones are easier to handle and more sturdily built, and will
last for three or four years.  Or you can build something out of metal
that you can mount on the box of your pickup.  Use leather-palmed
gloves when handling the rope.	It's pretty abrasive.

2.7  Bridle release

The Blanik's CG release mechchanism is an elegantly simple and
robust design, but it must be understood and managed properly to avoid
trouble.  ("Trouble" means premature release of one side at the beginning
of the ground roll, which can cause the glider to slew severely; or it
means release failure due to sitcking or jamming of the mechanism.  We
have seen neither, but have heard of this happening.)

Release mechanism.  The Blanik's release mechanism provides a
large pin that retracts flush with the fuselage when the release handle
is pulled.




Note in this illustration how the latch is constructed.

This is the port-side release mechanism; the front of the glider
is to your left, the back to your right.  Down is down...

At the left you see the hook attached to the ground launch bridle,
ready to be inserted in the release mechanism.

The release mechanism consists of four integrated components:

- The triangular base, bolted to the fuselage, which holds the
the cylindrical pin.

- The top rim of the base is "folded over" making a groove into which the
tip of the hook fits.  It is important that this groove be meticuously
cleaned regularly, as crud wanders into it, and crud, as we all know, is
sticky.  Stickiness hinders the hook from falling out freely, which we want
very much for it to do when we retract the pin with our left hand on
the yellow handle in the cockpit.  During the pre-launch check, the
hooks should be securely inserted into the release mechanisms on both
sides of the glider; and both should fall freely to the
ground when the release is pulled.

- The cylindrical pin is solid and very sturdy.  Your task is to make
sure that it is clean and well lubricated, so that pulling the release
handle is free and effortless except for the resistance of the spring
against which you pull.

- The little triangular gizmo at the 8 o'clock position is the back-
release.  Pull the release handle so that the pin retracts, slip the
hook into its slot, release the handle, and the pin is secure.  Test
the back release by cranking the hook counterclockwise; the gizmo will
rotate freely about 90 degrees and the tip of the hook will no longer
be within the slot at the top, and will fall away.

It is also possible to insert the hook by depressing the release and
rotating the hook counterclockwise into the slot at the top.



Here we see the hook fitted into the release mechanism, ready to roll.
Note the scratches on the fuselage, made during launches as the rope
becomes more vertical during climb, and note that the tip of the hook
must eventually fall out of the groove that holds it as the rope's
angle goes past 90 degrees.

The release on the starboard side is the mirror image, in case you
were wondering....

Copyright © 2000, 2002 Daniel L. Johnson; all rights reserved.